Multiple molecular chaperones complex with misfolded large oligomeric glycoproteins in the endoplasmic reticulum

The Essential Functions of Molecular Chaperones and Folding Enzymes in Maintaining Endoplasmic Reticulum Homeostasis

It has been estimated that up to one-third of the proteins encoded by the human genome enter the endoplasmic reticulum (ER) as extended polypeptide chains where they undergo covalent modifications, fold into their native structures, and assemble into oligomeric protein complexes. The fidelity of these processes is critical to support organellar, cellular, and organismal health, and is perhaps best underscored by the growing number of disease-causing mutations that reduce the fidelity of protein biogenesis in the ER. To meet demands encountered by the diverse protein clientele that mature in the ER, this organelle is populated with a cadre of molecular chaperones that prevent protein aggregation, facilitate protein disulfide isomerization, and lower the activation energy barrier of cis-trans prolyl isomerization. Components of the lectin (glycan-binding) chaperone system also reside within the ER and play numerous roles during protein biogenesis. In addition, the ER houses multiple homologs of select chaperones that can recognize and act upon diverse peptide signatures. Moreover, redundancy helps ensure that folding-compromised substrates are unable to overwhelm essential ER-resident chaperones and enzymes. In contrast, the ER in higher eukaryotic cells possesses a single member of the Hsp70, Hsp90, and Hsp110 chaperone families, even though several homologs of these molecules reside in the cytoplasm. In this review, we discuss specific functions of the many factors that maintain ER quality control, highlight some of their interactions, and describe the vulnerabilities that arise from the absence of multiple members of some chaperone families.

The ER chaperone, BIP protects Microglia from ER stress-mediated Apoptosis in Hyperglycemia

A major endoplasmic reticulum (ER) chaperone, binding of Immunoglobulin heavy chain protein (BIP) facilitates the assembly of newly synthesized proteins in the ER. Microglia vigorously respond to brain injuries and eliminate the damaged neuronal and apoptotic cells through phagocytosis in the central nervous system. However, the mechanism of BIP-mediated microglial function is not clear in hyperglycemia. We explored the molecular mechanism of BIP in microglial function during hyperglycemic conditions. Hyperglycemia was induced in mice by two consecutive intraperitoneal injections of streptozotocin (STZ 100/kg) and confirmed by measuring the blood glucose from day 2 to day 14. After 14 days of experimental hyperglycemia, mice were sacrificed and brains were collected for ER chaperone expression. In-vitro hyperglycemia was induced by exposing HMC3 cells to 25 mM glucose for 5 days and proteins involved in ER stress, apoptosis, and autophagy were analyzed. In hyperglycemic conditions, BIP protein expression was dramatically reduced in HMC3 cells, which led to increased apoptosis through the activation of CHOP and mitochondrial pro-apoptotic proteins (Bax, Bad, and cleaved caspase-3). The flow cytometry results indicate hyperglycemia-induced apoptosis and reactive oxygen species (ROS) production. Interestingly, the BIP inducer X restored the apoptosis in HMC3 cells by derepressing BIP expression and inhibiting ER stress. These results suggest that the ER chaperone BIP is required for the microglial function and protects from apoptosis in hyperglycemia. A better understanding of BIP's molecular mechanism and role in microglial function may contribute to developing novel therapies for microglia dysfunction-associated neurodegenerative diseases.

Prioritization of Candidate Biomarkers for Degenerative Aortic Stenosis through a Systems Biology-Based In-Silico Approach

Degenerative aortic stenosis is the most common valve disease in the elderly and is usually confirmed at an advanced stage when the only treatment is surgery. This work is focused on the study of previously defined biomarkers through systems biology and artificial neuronal networks to understand their potential role within aortic stenosis. The goal was generating a molecular panel of biomarkers to ensure an accurate diagnosis, risk stratification, and follow-up of aortic stenosis patients. We used in silico studies to combine and re-analyze the results of our previous studies and, with information from multiple databases, established a mathematical model. After this, we prioritized two proteins related to endoplasmic reticulum stress, thrombospondin-1 and endoplasmin, which have not been previously validated as markers for aortic stenosis, and analyzed them in a cell model and in plasma from human subjects. Large-scale bioinformatics tools allow us to extract the most significant results after using high throughput analytical techniques. Our results could help to prevent the development of aortic stenosis and open the possibility of a future strategy based on more specific therapies.

PDIA3/ERp57 promotes a matrix-rich secretome that stimulates fibroblast adhesion through CCN2

The matricellular glycoprotein thrombospondin1 (TSP1) has complex roles in the extracellular matrix and at cell surfaces, but relatively little is known about its intracellular associations prior to secretion. To search for novel intracellular interactions of TSP1 in situ, we carried out a biotin ligase-based TSP1 interactome screen and identified protein disulphide isomerase A3 (PDIA3/ERp57) as a novel candidate binding protein. In validation, TSP1 and PDIA3 were established to bind in vitro and to colocalise in the endoplasmic reticulum of human dermal fibroblasts (HDF). Loss of PDIA3 function, either by pharmacological inhibition in HDF or in Pdia3^-/- mouse embryo fibroblasts (Pdia3^-/-MEF), led to alterations in the composition of cell-derived ECM, involving changed abundance of fibronectin and TSP1, and was correlated with reduced cell spreading, altered organisation of F-actin and reduced focal adhesions. These cellular phenotypes of Pdia3^-/-MEF were normalised by exposure to conditioned medium (WTCM) or extracellular matrix (WTECM) from wild-type (WT)-MEF. Rescue depended on PDIA3 activity in WT-MEF, and was not prevented by immunodepletion of fibronectin. Heparin-binding proteins in WTCM were found to be necessary for rescue. Comparative quantitative tandem-mass-tag proteomics and functional assays on the heparin-binding secretomes of WT-MEF and Pdia3^-/- MEF identified multiple ECM and growth factor proteins to be down-regulated in the CM of Pdia3^-/- MEF. Of these, CCN2 was identified to be necessary for the adhesion-promoting activity of WTCM on Pdia3^-/- MEF and to bind TSP1. Thus, PDIA3 coordinates fibroblast production of an ECM-rich, pro-adhesive microenvironment, with implications for PDIA3 as a translational target.

The HSP90 Family: Structure, Regulation, Function, and Implications in Health and Disease

The mammalian HSP90 family of proteins is a cluster of highly conserved molecules that are involved in myriad cellular processes. Their distribution in various cellular compartments underlines their essential roles in cellular homeostasis. HSP90 and its co-chaperones orchestrate crucial physiological processes such as cell survival, cell cycle control, hormone signaling, and apoptosis. Conversely, HSP90, and its secreted forms, contribute to the development and progress of serious pathologies, including cancer and neurodegenerative diseases. Therefore, targeting HSP90 is an attractive strategy for the treatment of neoplasms and other diseases. This manuscript will review the general structure, regulation and function of HSP90 family and their potential role in pathophysiology.

ORP150-CHIP chaperone antagonism control BACE1-mediated amyloid processing

BACE1, a key protein involved in Alzheimer's progression, initiates Aβ42 generation that induce senile plaques in brain. However, the role of chaperone synergy or antagonism on BACE1-mediated amyloid processing is unknown. We have discovered that BACE1 as well as Aβ42 are antagonistically controlled by ER chaperone ORP150 and cellular chaperone CHIP. We have shown ORP150 as a chaperone interacts with and stabilizes BACE1 at post-translational level. Furthermore, ORP150 enhances BACE1-mediated amyloid processing thus masking CHIP-mediated BACE1 degradation. Conversely, siORP150 reversed the chaperone function of ORP150 resulting in BACE1 degradation. ORP150 and CHIP demonstrate antagonism under normal and stress conditions wherein they inversely regulate each other thus affecting BACE1 level. In conclusion, we have uncovered for the first time a phenomenon of chaperone antagonism on BACE1-mediated Aβ42 generation. Future strategy would require both suppression of ORP150 as well as activation of E3-ligase activity of CHIP that might prevent Aβ42 in Alzheimer's disease.

Expression of Endoplasmic Reticulum-Mediated Stress Proteins in FK506-Treated T-Lymphocytes

BACKGROUND

FK506-induced apoptotic endoplasmic reticulum (ER)-mediated stress protein expression was investigated in Jurkat human T-lymphocytes.

METHODS

The effect of FK506 on apoptosis and cell viability were examined. FK506-induced apoptosis was confirmed by nuclear fragmentation after DAPI staining. Expression of apoptotic ER-mediated stress proteins was examined by means of Western blotting of Grp78/BiP, Grp94, double-stranded RNA-dependent protein kinase (PKR)-like ER kinase (PERK), phosphor-PERK, CHOP/GADD153, and Bak. A flow cytometry analysis was performed after DAF-DA or DCF-DA staining. FK506-induced apoptosis was dose-dependent (10 nmol/L) and time-dependent (72 hours).

RESULTS

Grp78/BiP and Grp94 expressions were increased 36 hours after FK506 treatment. Increased phospho-PERK expression was observed 6 hours after FK506 treatment and peak activation of phospho-PERK was observed at 36 hours. CHOP/GADD153 expression was increased 48 hours after FK506 treatment. Expression of iNOS after FK506 treatment began to increase at 12 hours, peaked at 24 hours, and decreased after 36 hours.

CONCLUSIONS

From these results, we confirmed that FK506 induces apoptosis and acts dose- and time-dependently to decrease the viability of Jurkat cells through activation of apoptosis signaling and expression of apoptotic ER-mediated stress proteins.

Insights into the posttranslational structural heterogeneity of thyroglobulin and its role in the development, diagnosis, and management of benign and malignant thyroid diseases

Thyroglobulin (Tg) is the major glycoprotein produced by the thyroid gland, where it serves as a template for thyroid hormone synthesis and as an intraglandular store of iodine. Measurement of Tg levels in serum is of great practical importance in the follow-up of differentiated thyroid carcinoma (DTC), a setting in which elevated levels after total thyroidectomy are indicative of residual or recurrent disease. The most recent methods for serum Tg measurement are monoclonal antibody-based and are highly sensitive. However, major challenges remain regarding the interpretation of the results obtained with these immunometric methods, particularly in patients with endogenous antithyroglobulin antibodies or in the presence of heterophile antibodies, which may produce falsely low or high Tg values, respectively. The increased prevalence of antithyroglobulin antibodies in patients with DTC, as compared with the general population, raises the very pertinent possibility that tumor Tg may be more immunogenic. This inference makes sense, as the tumor microenvironment (tumor cells plus normal host cells) is characterized by several changes that could induce posttranslational modification of many proteins, including Tg. Attempts to understand the structure of Tg have been made for several decades, but findings have generally been incomplete due to technical hindrances to analysis of such a large protein (660 kDa). This review article will explore the complex structure of Tg and the potential role of its marked heterogeneity in our understanding of normal thyroid biology and neoplastic processes.

GRP78 and CHOP modulate macrophage apoptosis and the development of bleomycin-induced pulmonary fibrosis

Endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) have been associated with fibrotic lung disease, although exactly how they modulate this process remains unclear. Here we investigated the role of GRP78, the main UPR regulator, in an experimental model of lung injury and fibrosis. Grp78(+/-) , Chop(-/-) and wild type C57BL6/J mice were exposed to bleomycin by oropharyngeal intubation and lungs were examined at days 7 and 21. We demonstrate here that Grp78(+/-) mice were strongly protected from bleomycin-induced fibrosis, as shown by immunohistochemical analysis, collagen content and lung function measurements. In the inflammatory phase of this model, a reduced number of lung macrophages associated with an increased number of TUNEL-positive cells were observed in Grp78(+/-) mice. Dual immunohistochemical and in situ hybridization experiments showed that the macrophage population from the protected Grp78(+/-) mice was also strongly positive for cleaved caspase-3 and Chop mRNA, respectively. In contrast, the administration of bleomycin to Chop(-/-) mice resulted in increased quasi-static elastance and extracellular matrix deposition associated with an increased number of parenchymal arginase-1-positive macrophages that were negative for cleaved caspase-3. The data presented indicate that the UPR is activated in fibrotic lung tissue and strongly localized to macrophages. GRP78- and CHOP-mediated macrophage apoptosis was found to protect against bleomycin-induced fibrosis. Overall, we demonstrate here that the fibrotic response to bleomycin is dependent on GRP78-mediated events and provides evidence that macrophage polarization and apoptosis may play a role in this process. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

BiP and its nucleotide exchange factors Grp170 and Sil1: mechanisms of action and biological functions

BiP (immunoglobulin heavy-chain binding protein) is the endoplasmic reticulum (ER) orthologue of the Hsp70 family of molecular chaperones and is intricately involved in most functions of this organelle through its interactions with a variety of substrates and regulatory proteins. Like all Hsp70 family members, the ability of BiP to bind and release unfolded proteins is tightly regulated by a cycle of ATP binding, hydrolysis, and nucleotide exchange. As a characteristic of the Hsp70 family, multiple DnaJ-like co-factors can target substrates to BiP and stimulate its ATPase activity to stabilize the binding of BiP to substrates. However, only in the past decade have nucleotide exchange factors for BiP been identified, which has shed light not only on the mechanism of BiP-assisted folding in the ER but also on Hsp70 family members that reside throughout the cell. We will review the current understanding of the ATPase cycle of BiP in the unique environment of the ER and how it is regulated by the nucleotide exchange factors, Grp170 (glucose-regulated protein of 170kDa) and Sil1, both of which perform unanticipated roles in various biological functions and disease states.

Current understanding of the thrombospondin-1 interactome

The multifaceted action of thrombospondin-1 (TSP-1) depends on its ability to physically interact with different ligands, including structural components of the extracellular matrix, other matricellular proteins, cell receptors, growth factors, cytokines and proteases. Through this network, TSP-1 regulates the ligand activity, availability and structure, ultimately tuning the cell response to environmental stimuli in a context-dependent manner, contributing to physiological and pathological processes. Complete mapping of the TSP-1 interactome is needed to understand its diverse functions and to lay the basis for the rational design of TSP-1-based therapeutic approaches. So far, large-scale approaches to identify TSP-1 ligands have been rarely used, but many interactions have been identified in small-scale studies in defined biological systems. This review, based on information from protein interaction databases and the literature, illustrates current knowledge of the TSP-1 interactome map.

Orchestration of secretory protein folding by ER chaperones

The endoplasmic reticulum is a major compartment of protein biogenesis in the cell, dedicated to production of secretory, membrane and organelle proteins. The secretome has distinct structural and post-translational characteristics, since folding in the ER occurs in an environment that is distinct in terms of its ionic composition, dynamics and requirements for quality control. The folding machinery in the ER therefore includes chaperones and folding enzymes that introduce, monitor and react to disulfide bonds, glycans, and fluctuations of luminal calcium. We describe the major chaperone networks in the lumen and discuss how they have distinct modes of operation that enable cells to accomplish highly efficient production of the secretome. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Intravitreal transplantation of human umbilical cord blood stem cells protects rats from traumatic optic neuropathy

Objectives

To treat traumatic optic neuropathy (TON) with transplantation of human umbilical cord blood stem cells (hUCBSC) and explore how transplanted stem cells participate in the neuron repairing process.

Methods

A total of 195 Sprague-Dawley rats were randomly assigned to three groups: sham-surgery, optic nerve injury, and stem cell transplant group. Optic nerve injury was established in rats by directly clamping the optic nerve for 30 seconds. hUCBSC was microinjected into the vitreous cavity of injured rats. Optic nerve function was evaluated by flash visual evoked potentials (F-VEP). Apoptosis in retina tissues was detected by TUNEL staining. GRP78 and CHOP gene expression was measured by RT-PCR.

Results

After injury, transplantation of hUCBSC significantly blunted a reduction in optic nerve function indicated by smaller decreases in amplitude and smaller increases in peak latency of F-VEP waveform compared to the injury alone group. Also, significant more in retinal ganglion cell (RGC) count and less in RGC apoptosis were detected after transplantation compared to injured rats. The protective effect correlated with upregulated GRP78 and downregulated CHOP mRNA expression.

Conclusion

Intravitreal transplantation of hUCBSCs significantly blunted a reduction in optic nerve function through increasing RGC survival and decreasing retinal cell apoptosis. The protective role of transplantation was associated with upregulation of GRP78 expression and downregulation of CHOP expression in retinal cells.

Unfolded protein responses with or without unfolded proteins?

The endoplasmic reticulum (ER) is the site of secretory protein biogenesis. The ER quality control (QC) machinery, including chaperones, ensures the correct folding of secretory proteins. Mutant proteins and environmental stresses can overwhelm the available QC machinery. To prevent and resolve accumulation of misfolded secretory proteins in the ER, cells have evolved integral membrane sensors that orchestrate the Unfolded Protein Response (UPR). The sensors, Ire1p in yeast and IRE1, ATF6, and PERK in metazoans, bind the luminal ER chaperone BiP during homeostasis. As unfolded secretory proteins accumulate in the ER lumen, BiP releases, and the sensors activate. The mechanisms of activation and attenuation of the UPR sensors have exhibited unexpected complexity. A growing body of data supports a model in which Ire1p, and potentially IRE1, directly bind unfolded proteins as part of the activation process. However, evidence for an unfolded protein-independent mechanism has recently emerged, suggesting that UPR can be activated by multiple modes. Importantly, dysregulation of the UPR has been linked to human diseases including Type II diabetes, heart disease, and cancer. The existence of alternative regulatory pathways for UPR sensors raises the exciting possibility for the development of new classes of therapeutics for these medically important proteins.

A thrombospondin-dependent pathway for a protective ER stress response

Thrombospondin (Thbs) proteins are induced in sites of tissue damage or active remodeling. The endoplasmic reticulum (ER) stress response is also prominently induced with disease where it regulates protein production and resolution of misfolded proteins. Here we describe a function for Thbs as ER-resident effectors of an adaptive ER stress response. Thbs4 cardiac-specific transgenic mice were protected from myocardial injury, whereas Thbs4(-/-) mice were sensitized to cardiac maladaptation. Thbs induction produced a unique profile of adaptive ER stress response factors and expansion of the ER and downstream vesicles. Thbs bind the ER lumenal domain of activating transcription factor 6α (Atf6α) to promote its nuclear shuttling. Thbs4(-/-) mice showed blunted activation of Atf6α and other ER stress-response factors with injury, and Thbs4-mediated protection was lost upon Atf6α deletion. Hence, Thbs can function inside the cell during disease remodeling to augment ER function and protect through a mechanism involving regulation of Atf6α.

Mesencephalic astrocyte-derived neurotrophic factor protects the heart from ischemic damage and is selectively secreted upon sarco/endoplasmic reticulum calcium depletion

The endoplasmic reticulum (ER) stress protein mesencephalic astrocyte-derived neurotrophic factor (MANF) has been reported to protect cells from stress-induced cell death before and after its secretion; however, the conditions under which it is secreted are not known. Accordingly, we examined the mechanism of MANF release from cultured ventricular myocytes and HeLa cells, both of which secrete proteins via the constitutive pathway. Although the secretion of proteins via the constitutive pathway is not known to increase upon changes in intracellular calcium, MANF secretion was increased within 30 min of treating cells with compounds that deplete sarcoplasmic reticulum (SR)/ER calcium. In contrast, secretion of atrial natriuretic factor from ventricular myocytes was not increased by SR/ER calcium depletion, suggesting that not all secreted proteins exhibit the same characteristics as MANF. We postulated that SR/ER calcium depletion triggered MANF secretion by decreasing its retention. Consistent with this were co-immunoprecipitation and live cell, zero distance, photo affinity cross-linking, demonstrating that, in part, MANF was retained in the SR/ER via its calcium-dependent interaction with the SR/ER-resident protein, GRP78 (glucose-regulated protein 78 kDa). This unusual mechanism of regulating secretion from the constitutive secretory pathway provides a potentially missing link in the mechanism by which extracellular MANF protects cells from stresses that deplete SR/ER calcium. Consistent with this was our finding that administration of recombinant MANF to mice decreased tissue damage in an in vivo model of myocardial infarction, a condition during which ER calcium is known to be dysregulated, and MANF expression is induced.

Endoplasmic reticulum chaperone glucose regulated protein 170-Pokemon complexes elicit a robust antitumor immune response in vivo

Previous evidence suggested that the stress protein grp170 can function as a highly efficient molecular chaperone, binding to large protein substrates and acting as a potent vaccine against specific tumors when purified from the same tumor. In addition, Pokemon can be found in almost all malignant tumor cells and is regarded to be a promising candidate for the treatment of tumors. However, the potential of the grp170-Pokemon chaperone complex has not been well described. In the present study, the natural chaperone complex between grp170 and the Pokemon was formed by heat shock, and its immunogenicity was detected by ELISPOT and (51)Cr-release assays in vitro and by tumor bearing models in vivo. Our results demonstrated that the grp170-Pokemon chaperone complex could elicit T cell responses as determined by ELISPOT and (51)Cr-release assays. In addition, immunized C57BL/6 mice were challenged with subcutaneous (s.c.) injection of Lewis cancer cells to induce primary tumors. Treatment of mice with the grp170-Pokemon chaperone complex also significantly inhibited tumor growth and prolonged the life span of tumor-bearing mice. Our results indicated that the grp170-Pokemon chaperone complex might represent a powerful approach to tumor immunotherapy and have significant potential for clinical application.

Protein disulfide isomerases contribute differentially to the endoplasmic reticulum-associated degradation of apolipoprotein B and other substrates

ER-associated degradation (ERAD) rids the early secretory pathway of misfolded or misprocessed proteins. Some members of the protein disulfide isomerase (PDI) family appear to facilitate ERAD substrate selection and retrotranslocation, but a thorough characterization of PDIs during the degradation of diverse substrates has not been undertaken, in part because there are 20 PDI family members in mammals. PDIs can also exhibit disulfide redox, isomerization, and/or chaperone activity, but which of these activities is required for the ERAD of different substrate classes is unknown. We therefore examined the fates of unique substrates in yeast, which expresses five PDIs. Through the use of a yeast expression system for apolipoprotein B (ApoB), which is disulfide rich, we discovered that Pdi1 interacts with ApoB and facilitates degradation through its chaperone activity. In contrast, Pdi1's redox activity was required for the ERAD of CPY* (a misfolded version of carboxypeptidase Y that has five disulfide bonds). The ERAD of another substrate, the alpha subunit of the epithelial sodium channel, was Pdi1 independent. Distinct effects of mammalian PDI homologues on ApoB degradation were then observed in hepatic cells. These data indicate that PDIs contribute to the ERAD of proteins through different mechanisms and that PDI diversity is critical to recognize the spectrum of potential ERAD substrates.

The thrombospondins

Thrombospondins are evolutionarily conserved, calcium-binding glycoproteins that undergo transient or longer-term interactions with other extracellular matrix components. They share properties with other matrix molecules, cytokines, adaptor proteins, and chaperones, modulate the organization of collagen fibrils, and bind and localize an array of growth factors or proteases. At cell surfaces, interactions with an array of receptors activate cell-dependent signaling and phenotypic outcomes. Through these dynamic, pleiotropic, and context-dependent pathways, mammalian thrombospondins contribute to wound healing and angiogenesis, vessel wall biology, connective tissue organization, and synaptogenesis. We overview the domain organization and structure of thrombospondins, key features of their evolution, and their cell biology. We discuss their roles in vivo, associations with human disease, and ongoing translational applications. In many respects, we are only beginning to appreciate the important roles of these proteins in physiology and pathology.

The endoplasmic reticulum protein folding factory and its chaperones: new targets for drug discovery?

Cytosolic heat shock proteins have received significant attention as emerging therapeutic targets. Much of this excitement has been triggered by the discovery that HSP90 plays a central role in the maintenance and stability of multifarious oncogenic membrane receptors and their resultant tyrosine kinase activity. Numerous studies have dealt with the effects of small molecules on chaperone- and stress-related pathways of the endoplasmic reticulum (ER). However, unlike cytosolic chaperones, relatively little emphasis has been placed upon translational avenues towards targeting of the ER for inhibition of folding/secretion of disease-promoting proteins. Here, we summarise existing small molecule inhibitors and potential future targets of ER chaperone-mediated inhibition. Client proteins of translational relevance in disease treatment are outlined, alongside putative future disease treatment modalities based on ER-centric targeted therapies. Particular attention is paid to cancer and autoimmune disorders via the effects of the GRP94 inhibitor geldanamycin and its population of client proteins, overloading of the unfolded protein response, and inhibition of members of the IL-12 family of cytokines by celecoxib and non-coxib analogues.

Tumour secreted grp170 chaperones full-length protein substrates and induces an adaptive anti-tumour immune response in vivo

PURPOSE

We employed a grp170-secreting tumour cell system to determine whether tumour cells engineered to secrete grp170 generate an antitumour-specific immune response. Further, we examine the possibility that secreted grp170 can bind to and co-transport out of tumour cells full-length tumour antigens that may play a role in the anti-tumour immune response.

MATERIALS AND METHODS

Wild type Colon-26 and Colon-26 engineered to secrete grp170 were subcutaneously inoculated into BALB/c mice. Tumour growth was monitored, and variations in immunoregulatory mechanisms were evaluated using immunohistochemistry, lymphocyte depletion, ELISpot assays, and Western blot analysis.

RESULTS

Immunisation of animals with grp170-secreting tumour cells results in rejection of the tumour by induction of antigen-specific, CD8-dependent immune responses. The secreted grp170 is able to deliver full-length tumour antigens to the tumour microenvironment, thus making them available for uptake by antigen presenting cells (APCs) to initiate tumour-specific immune responses.

CONCLUSIONS

These data parallel our studies showing that hsp110 or grp170 are able to chaperone full-length proteins, and when complexed with protein antigens and used as vaccines, these complexes elicit immune responses in vivo against the protein antigens. This cell-based approach has the potential to be utilised as a tumour-specific vaccine in tumours of various histological origins.

Vaccination with a chaperone complex based on PSCA and GRP170 adjuvant enhances the CTL response and inhibits the tumor growth in mice

Increasing knowledge demonstrate that prostate stem cell antigen (PSCA) is a promising candidate for immunotherapy of advanced prostate cancer. However, tumor escape with down-regulation of target antigens may limit the susceptibility of tumor cells to the immune attack. Concomitant generation of T-cell responses against several immunodominant antigens may circumvent this potential drawback. In this study, we prepared the chaperone complex vaccine based on PSCA and GRP170, and utilized it to immunize the C57BL/6 mice. In addition, the T-cell response was monitored with ELISPOT and (51)Cr-release assays, and the tumor growth and the life span of tumor-bearing mice were assessed. The results demonstrated the chaperone complex based on PSCA and GRP170 could enhance the T-cell mediate immune responses, which significantly inhibited the tumor growth and prolonged the life span of tumor-bearing mice. In conclusion, our findings supported the strategy of chaperone complex, based on PSCA and GRP170, could be an effective treatment for prostate cancer therapy.

BiP availability distinguishes states of homeostasis and stress in the endoplasmic reticulum of living cells

BiP availability represents a powerful tool for reporting global secretory protein misfolding levels and investigating the molecular events of ER stress in single cells, independent of traditional UPR markers.

Accumulation of misfolded secretory proteins causes cellular stress and induces the endoplasmic reticulum (ER) stress pathway, the unfolded protein response (UPR). Although the UPR has been extensively studied, little is known about the molecular changes that distinguish the homeostatic and stressed ER. The increase in levels of misfolded proteins and formation of complexes with chaperones during ER stress are predicted to further crowd the already crowded ER lumen. Surprisingly, using live cell fluorescence microscopy and an inert ER reporter, we find the crowdedness of stressed ER, treated acutely with tunicamycin or DTT, either is comparable to homeostasis or significantly decreases in multiple cell types. In contrast, photobleaching experiments revealed a GFP-tagged variant of the ER chaperone BiP rapidly undergoes a reversible quantitative decrease in diffusion as misfolded proteins accumulate. BiP mobility is sensitive to exceptionally low levels of misfolded protein stressors and can detect intermediate states of BiP availability. Decreased BiP availability temporally correlates with UPR markers, but restoration of BiP availability correlates less well. Thus, BiP availability represents a novel and powerful tool for reporting global secretory protein misfolding levels and investigating the molecular events of ER stress in single cells, independent of traditional UPR markers.

The trafficking protein Tmed2/p24beta(1) is required for morphogenesis of the mouse embryo and placenta

During vesicular transport between the endoplasmic reticulum and the Golgi, members of the TMED/p24 protein family form hetero-oligomeric complexes that facilitate protein-cargo recognition as well as vesicle budding. In addition, they regulate each other's level of expression. Despite analyses of TMED/p24 protein distribution in mammalian cells, yeast, and C. elegans, little is known about the role of this family in vertebrate embryogenesis. We report the presence of a single point mutation in Tmed2/p24beta(1) in a mutant mouse line, 99J, identified in an ENU mutagenesis screen for recessive developmental abnormalities. This mutation does not affect Tmed2/p24beta(1) mRNA levels but results in loss of TMED2/p24beta(1) protein. Prior to death at mid-gestation, 99J homozygous mutant embryos exhibit developmental delay, abnormal rostral-caudal elongation, randomized heart looping, and absence of the labyrinth layer of the placenta. We find that Tmed2/p24beta(1) is normally expressed in tissues showing morphological defects in 99J mutant embryos and that these affected tissues lack the TMED2/p24beta(1) oligomerization partners, TMED7/p24gamma(3) and TMED10/p24delta(1). Our data reveal a requirement for TMED2/p24beta(1) protein in the morphogenesis of the mouse embryo and placenta.

Altered synthesis of cartilage-specific proteoglycans by mutant human cartilage oligomeric matrix protein

Background

The mechanism by which mutant cartilage oligomeric matrix protein (COMP) induces a pseudoachondroplasia phenotype remains unknown, and the reason why a mutation of a minor protein of the growth plate cartilage causes total disruption of endochondral bone formation has not yet been determined. The current study was performed to investigate the effects of mutated COMP on the synthesis of the cartilage-specific major matrix proteins of Swarm rat chondrosarcoma chondrocytes.

Methods

The Swarm rat chondrosarcoma chondrocytes transfected with a chimeric construct, which consisted of a mutant gene of human COMP and an amino acid FLAG tag sequence, were cultured in agarose gel. Formation of extracellular proteoglycan and type-II collagen by the cells was evaluated by immunohistochemical staining and measuring the ³⁵S-sulfate incorporation.

Results

No difference was observed for the detection of type-II collagen among the cell lines expressing mutant COMP and the control cell lines. Histochemical staining of sulfated proteoglycans with safranin-O showed that lesser amounts of proteoglycans were incorporated into the extracellular matrix of the chondrocytes transfected with the mutant gene. ³⁵S-sulfate incorporation into the cell/matrix fractions demonstrated markedly lower radiolabel incorporation, as compared to that of the control cells.

Conclusions

Mutation of COMP has an important impact on the processing of proteoglycans, rather than type-II collagen, in the three-dimensional culture of Swarm rat chondrosarcoma chondrocytes.

Protein disulfide isomerase: a critical evaluation of its function in disulfide bond formation

Disulfide bond formation is probably involved in the biogenesis of approximately one third of human proteins. A central player in this essential process is protein disulfide isomerase or PDI. PDI was the first protein-folding catalyst reported. However, despite more than four decades of study, we still do not understand much about its physiological mechanisms of action. This review examines the published literature with a critical eye. This review aims to (a) provide background on the chemistry of disulfide bond formation and rearrangement, including the concept of reduction potential, before examining the structure of PDI; (b) detail the thiol-disulfide exchange reactions that are catalyzed by PDI in vitro, including a critical examination of the assays used to determine them; (c) examine oxidation and reduction of PDI in vivo, including not only the role of ERo1 but also an extensive assessment of the role of glutathione, as well as other systems, such as peroxide, dehydroascorbate, and a discussion of vitamin K-based systems; (d) consider the in vivo reactions of PDI and the determination and implications of the redox state of PDI in vivo; and (e) discuss other human and yeast PDI-family members.

Induction of BiP, an ER-resident protein, prevents the neuronal death induced by transient forebrain ischemia in gerbil

Endoplasmic reticulum (ER) stress, which is caused by the accumulation of unfolded proteins in the ER lumen, is associated with stroke and neurodegenerative diseases such as Parkinson's and Alzheimer's diseases. We evaluated the effect of a selective inducer of immunoglobulin heavy chain binding protein (BiP) (BiP inducer X; BIX) against both tunicamycin-induced cell death (in SH-SY5Y cells) and the effects of global transient forebrain ischemia (in gerbils). BIX significantly induced BiP expression both in vitro and in vivo. Pretreatment with BIX at 2 or 5 microM reduced the cell death induced by tunicamycin in SH-SY5Y cells. In gerbils subjected to forebrain ischemia, prior treatment with BIX (intracerebroventricular injection at 10 or 40 microg) protected against cell death and decreased TUNEL-positive cells in the hippocampal CA1 subfield. These findings indicate that this selective inducer of BiP could be used to prevent the neuronal damage both in vitro and in vivo.

Transcriptional profiling of gene expression changes in a PACE-transfected CHO DUKX cell line secreting high levels of rhBMP-2

Chinese hamster ovary (CHO) cells are widely used in the biopharmaceutical industry for the production of recombinant human proteins including complex polypeptides such as recombinant human bone morphogenic protein 2 (rhBMP-2). Large-scale manufacture of rhBMP-2 has associated production difficulties resulting from incomplete processing of the recombinant human protein due to insufficient endogenous levels of the paired basic amino acid cleaving enzyme (PACE) in CHO. In order to resolve this issue, CHO DUKX cells expressing rhBMP-2 were transfected with the soluble version of human PACE (PACEsol) resulting in improved amino-terminal homogeneity and a fourfold increase in rhBMP-2 productivity. In this article, we present a microarray expression profile analysis comparing the parental lineage to the higher producing subclone co-expressing PACEsol using a proprietary CHO-specific microarray. Using this technology we observed 1,076 significantly different genes in the high-productivity cells co-expressing PACEsol. Following further analysis of the differentially expressed genes, the Unfolded Protein Response (UPR) component of the endoplasmic reticulum stress response pathway was identified as a key candidate for effecting increased productivity in this cell system. Several additional ER- and Golgi-localised proteins were identified which may also contribute to this effect. The results presented here support the use of large-scale microarray expression profiling as a viable and valuable route towards understanding the behaviour of bioprocess cultures in vitro.

The human PDI family: versatility packed into a single fold

The enzymes of the protein disulfide isomerase (PDI) family are thiol-disulfide oxidoreductases of the endoplasmic reticulum (ER). They contain a CXXC active-site sequence where the two cysteines catalyze the exchange of a disulfide bond with or within substrates. The primary function of the PDIs in promoting oxidative protein folding in the ER has been extended in recent years to include roles in other processes such as ER-associated degradation (ERAD), trafficking, calcium homeostasis, antigen presentation and virus entry. Some of these functions are performed by non-catalytic members of the family that lack the active-site cysteines. Regardless of their function, all human PDIs contain at least one domain of approximately 100 amino acid residues with structural homology to thioredoxin. As we learn more about the individual proteins of the family, a complex picture is emerging that emphasizes as much their differences as their similarities, and underlines the versatility of the thioredoxin fold. Here, we primarily explore the diversity of cellular functions described for the human PDIs.

Glucose-depleted medium reduces the collagen content of human skin fibroblast cultures

Glucose deprivation appeared to be a factor which induces oxygen regulated protein (ORP) 150 expression in the human skin fibroblasts cultures. Whereas glucose deprivation resulted in a slight (statistically insignificant) decrease of protein content in these cultures, a marked decrease of collagen content was observed, resulting in a distinct reduction of hydroxyproline: protein ratio. Furthermore, the appearance of ORP150 in glucose-deprived cultures coexisted with an increase of gelatinolytic activity and slight reduction in the expression of insulin-like growth factor-I (IGF-I) receptor. Since IGF-I is a main stimulator of collagen synthesis, the reduction in the expression of IGF-I receptor may result in a decrease of collagen synthesis. It is suggested that ORP 150 is a chaperon, which protects intracellular proteins against proteolytic effects exerted by hypoxia or glucose shortage. Since the total amount of protein in fibroblast cultures did not change much, it appears that collagen (in contrast to other proteins) was not efficiently protected. The decrease in collagen synthesis and the enhancement of collagen degradation by gelatinases may result in distinct reduction of collagen content in glucose-deprived fibroblast cultures.

Neurospora crassa FKBP22 Is a Novel ER Chaperone and Functionally Cooperates with BiP

FK506 binding proteins (FKBPs) belong to the family of peptidyl prolyl cis-trans isomerases (PPIases) catalyzing the cis/trans isomerisation of Xaa-Pro bonds in oligopeptides and proteins. FKBPs are involved in folding, assembly and trafficking of proteins. However, only limited knowledge is available about the roles of FKBPs in the endoplasmic reticulum (ER) and their interaction with other proteins. Here we show the ER located Neurospora crassa FKBP22 to be a dimeric protein with PPIase and a novel chaperone activity. While the homodimerization of FKBP22 is mediated by its carboxy-terminal domain, the amino-terminal domain is a functional FKBP domain. The chaperone activity is mediated by the FKBP domain but is exhibited only by the full-length protein. We further demonstrate a direct interaction between FKBP22 and BiP, the major Hsp70 chaperone in the ER. The binding to BiP is mediated by the FKBP domain of FKBP22. Interestingly BiP enhances the chaperone activity of FKBP22. Both proteins form a stable complex with an unfolded substrate protein and thereby prevent its aggregation. These results suggest that BiP and FKBP22 form a folding helper complex with a high chaperoning capacity in the ER of Neurospora crassa.

FKBP22 is part of chaperone/folding catalyst complexes in the endoplasmic reticulum ofNeurospora crassa

FKBP22 is a dimeric protein in the lumen of the endoplasmic reticulum, which exhibits a chaperone as well as a PPIase activity. It binds via its FK506 binding protein (FKBP) domain directly to the Hsp70 chaperone BiP that stimulates the chaperone activity of FKBP22. Here we demonstrate additionally the association of FKBP22 with the molecular chaperones and folding catalysts Grp170, alpha-subunit of glucosidase II, PDI, ERp38, and CyP23. These proteins are associated with FKBP22 in at least two protein complexes. Furthermore, we report an essential role for FKBP22 in the development of microconidiophores in Neurospora crassa.

Oxidoreductase interactions include a role for ERp72 engagement with mutant thyroglobulin from the rdw/rdw rat dwarf

Newly synthesized thyroglobulin (Tg), the secretory glycoprotein that serves as precursor in thyroid hormone synthesis, normally forms transient covalent protein complexes with oxidoreductases of the endoplasmic reticulum (ER). The Tg-G2320R mutation is responsible for congenital hypothyroidism in rdw/rdw rats, in which a lack of secondary thyroid enlargement (goiter) implicates death of thyrocytes as part of disease pathogenesis. We found that mutant Tg-G2320R was retained within the ER with no detectable synthesis of thyroxine, had persistent exposure of free cysteine thiols, and was associated with activated ER stress response but incomplete ER-associated degradation (ERAD). Tg-G2320R associated with multiple ER resident proteins, most notably ERp72, including covalent Tg-ERp72 interactions. In PC Cl3 thyrocytes, inducible overexpression of ERp72 increased the ability of cells to maintain Tg cysteines in a reduced state. Noncovalent interactions of several ER chaperones with newly synthesized Tg-G2320R diminished over time in parallel with ERAD of the mutant protein, yet a small ERAD-resistant Tg fraction remained engaged in covalent association with ERp72 even 2 days post-synthesis. Such covalent protein aggregates may set the stage for apoptotic thyrocyte cell death, preventing thyroid goiter formation in rdw/rdw rats.

Levels of Specific Peptide-HLA Class I Complex Predicts Tumor Cell Susceptibility to CTL Killing

Recognition of tumor-associated Ags (TAAs) on tumor cells by CTLs and the subsequent tumor cell death are assumed to be dependent on TAA protein expression and to correlate directly with the level of peptide displayed in the binding site of the HLA class I molecule. In this study we evaluated whether the levels of Her-2/neu protein expression on human tumor cell lines directly correlate with HLA-A*0201/Her2/neu peptide presentation and CTL recognition. We developed a TCR mimic (TCRm) mAb designated 1B8 that specifically recognizes the HLA-A2.1/Her2/neu peptide (369-377) (Her2(369)-A2) complex. TCRm mAb staining intensity varied for the five human tumor cell lines analyzed, suggesting quantitative differences in levels of the Her2(369)-A2 complex on these cells. Analysis of tumor cell lines pretreated with IFN-gamma and TNF-alpha for Her2/neu protein and HLA-A2 molecule expression did not reveal a direct correlation between the levels of Her2/neu Ag, HLA-A2 molecule, and Her2(369)-A2 complex expression. However, compared with untreated cells, cytokine-treated cell lines showed an increase in Her2(369)-A2 epitope density that directly correlated with enhanced tumor cell death (p = 0.05). Although a trend was observed between tumor cell lysis and the level of the Her2(369)-A2 complex for untreated cells, the association was not significant. These findings suggest that tumor cell susceptibility to CTL-mediated lysis may be predicted based on the level of specific peptide-MHC class I expression rather than on the total level of TAA expression. Further, these studies demonstrate the potential of the TCRm mAb for validation of endogenous HLA-peptide epitopes on tumor cells.

Multiple mechanisms limit the duration of wakefulness in Drosophila brain

The functions of sleep and what controls it remain unanswered biological questions. According to the two-process model, a circadian process and a homeostatic process interact to regulate sleep. While progress has been made in understanding the molecular and cellular functions of the circadian process, the mechanisms of the homeostatic process remain undiscovered. We use the recently established sleep model system organism Drosophila melanogaster to examine dynamic changes in gene expression during sleep and during prolonged wakefulness in the brain. Our experimental design controls for circadian processes by killing animals at three matched time points from the beginning of the consolidated rest period [Zeitgeber time (ZT) 14)] under two conditions, sleep deprived and spontaneously sleeping. Using ANOVA at a false discovery rate of 5%, we have identified 252 genes that were differentially expressed between sleep-deprived and control groups in the Drosophila brain. Using linear trends analysis, we have separated the significant differentially expressed genes into nine temporal expression patterns relative to a common anchor point (ZT 14). The most common expression pattern is a decrease during extended wakefulness but no change during spontaneous sleep (n = 114). Genes in this category were involved in protein production (n = 47), calcium homeostasis, and membrane excitability (n = 5). Multiple mechanisms, therefore, act to limit wakefulness. In addition, by studying the effects of the mechanical stimulus used in our deprivation studies during the period when the animals are predominantly active, we provide evidence for a previously unappreciated role for the Drosophila immune system in the brain response to stress.

Oxidative stress response of European flounder (Platichthys flesus) to cadmium determined by a custom cDNA microarray

The monitoring of the impact of chemical pollutants upon marine ecosystems commonly employs a multi-biomarker approach. Functional genomics, using cDNA microarrays, allows for a comprehensive view of how an organism is responding to an exposure, with respect to changes in gene expression. Differentially expressed mRNAs were first isolated from livers of European flounder by means of suppressive, subtractive hybridisation. A clone set containing a total of 284 different potentially differentially expressed mRNAs was produced, of which 84 were tentatively identified. These were combined with previously cloned known stress genes isolated by degenerate PCR to produce a custom 500-clone microarray platform with each clone arrayed to four spots. Subsequent array experiments using cadmium-treated flounder detected up-regulation of 27 transcripts, including Cu/Zn superoxide dismutase, thioredoxin, a peroxiredoxin and a glutathione-S-transferase, reflecting oxidative stress in exposed flounder, while CYP1A expression was down-regulated. These changes were confirmed by real-time PCR. The array experiment highlighted a number of candidate genes for further analysis as potential novel biomarkers of cadmium exposure and demonstrated the applicability of the custom microarray approach in the study of the effects of toxicants.

Multiple epiphyseal dysplasia mutations in MATN3 cause misfolding of the A-domain and prevent secretion of mutant matrilin-3

Multiple epiphyseal dysplasia (MED) is a relatively common skeletal dysplasia that can present in childhood with a variable phenotype of short stature and pain and stiffness in the large joints, and often progresses to early-onset osteoarthritis in adulthood. Mutations in the matrilin-3 gene (MATN3) have recently been shown to underlie some forms of autosomal dominant MED. To date all MED mutations in matrilin-3 cluster in the single A-domain, suggesting that they may disrupt the structure and/or function of this important domain. To determine the effects of MATN3 mutations on the structure and function of matrilin-3 we expressed both normal and mutant matrilin-3 in mammalian cells. Wild-type (wt) matrilin-3 was efficiently secreted into conditioned medium, whereas mutant matrilin-3 was retained and accumulated within the cell. Furthermore, when the mutant A-domains were examined individually, they existed primarily in an unfolded conformation. Co-immunoprecipitation experiments demonstrated that the mutant A-domains were specifically associated with ERp72, a chaperone protein known to be involved in mediating disulfide bond formation. Light microscopy of cartilage from an MED patient with a MATN3 mutation showed the presence of intracellular material within the chondrocytes, whilst the overall matrix appeared normal. On electron micrographs, the inclusions noted at the light microscopy level appeared to be dilated cisternae of rough endoplasmic reticulum and immunohistochemical analysis confirmed that the retained protein was matrilin-3. In summary, the data presented in this paper suggest that MED caused by MATN3 mutations is the result of an intracellular retention of the mutant protein.

Monitoring chaperone engagement of substrates in the endoplasmic reticulum of live cells

The folding environment in the endoplasmic reticulum (ER) depends on multiple abundant chaperones that function together to accommodate a range of substrates. The ways in which substrate engagement shapes either specific chaperone dynamics or general ER attributes in vivo remain unknown. In this study, we have evaluated how changes in substrate flux through the ER influence the diffusion of both the lectin chaperone calreticulin and an inert reporter of ER crowdedness. During acute changes in substrate load, the inert probe revealed no changes in ER organization, despite significant changes in calreticulin dynamics. By contrast, inhibition of the lectin chaperone system caused rapid changes in the ER environment that could be reversed over time by easing new substrate burden. Our findings provide insight into the normal organization and dynamics of an ER chaperone and characterize the capacity of the ER to maintain homeostasis during acute changes in chaperone activity and availability.

Multi-chaperone complexes regulate the folding of interferon-gamma in the endoplasmic reticulum

The quality control mechanisms directing the folding of cytokines in the endoplasmic reticulum (ER) are poorly understood. We have investigated ER chaperone usage by the cytokine interferon-gamma (IFN-gamma). ATP-depletion or inhibition of N-glycosylation was found to cause IFN-gamma to accumulate into detergent-insoluble aggregates in the ER. Six chaperones, GRP94, GRP78, ERp72, PDI, CaBP1/P5 and CRT were found to associate with IFN-gamma during its steady state folding. Interaction of the five first chaperones with IFN-gamma was regulated co-ordinately by ATP. These chaperones were recently reported to be part of a multi-chaperone complex involved in the folding of complex, multi-subunit proteins. Our data suggest that also proteins with a relatively simple quaternary structure such as cytokines may fold in association with this complex. In addition, we identified calreticulin as the major chaperone interacting with IFN-gamma, and the related class II cytokine interleukin-10, during heat-shock in vivo. IFN-gamma was maintained in a folding-competent form by calreticulin during heat-shock and released during subsequent recovery at 37 degrees C. This interaction was observed in both recombinant (CHO-F11) and natural producer cells (Jurkat, NK-92MI) of IFN-gamma. Since cytokines such as IFN-gamma and IL-10 are frequently produced in the course of inflammatory conditions associated with fever, the thermo-protective effect of calreticulin may constitute a previously unrecognized component of the cellular cytokine production machinery, of likely relevance in sustaining cytokine folding and secretion in pathophysiological conditions.

Chaperoning function of stress protein grp170, a member of the hsp70 superfamily, is responsible for its immunoadjuvant activity

When used as vaccines, tumor-derived stress proteins can elicit antitumor immune responses. For members of the hsp70 superfamily, like grp170, this seems to be due to (a) the chaperoning of antigenic peptide by the stress protein and (b) the binding of the stress protein to receptor(s) on antigen-presenting cells (APC) and subsequent antigen presentation. This suggests that domains exist on the stress protein for each function. In this study, we determine the ability of grp170 and its structural domains to (a) bind to and present melanoma-associated antigen gp100 to the immune system and (b) to bind to receptors on APCs. A direct correlation between chaperone function, binding to APCs in a receptor-like manner, and antitumor immunity was observed. Two mutants that share no common sequence, yet are both effective in their antitumor activities, compete with one another for APC binding. Studies of other members of the hsp70 superfamily, hsp110 and hsp70, or their domain deletion mutants, further confirmed that APC binding segregates with chaperoning function and not sequence. Therefore, these studies suggest that molecular chaperoning is involved in stress protein interactions with APCs, antigen binding, and in eliciting antitumor immunity, thus bridging this ancient function of stress proteins in prokaryotes to their ability to elicit immunity in higher organisms.

The effect of hypoxia on the expression of 150 kDa oxygen-regulated protein (ORP 150) in HeLa cells

Correct protein folding is an important factor, for the translocation of newly synthesised proteins to specific subcellular compartments, extracellular matrix or to biological fluids. This process is regulated by a group of specific proteins, referred to as chaperones. Many stress conditions, such as oxygen or glucose deprivation, slow down the folding process and cause accumulation of unfolded/misfolded proteins in the cell. Molecular chaperones are induced in these conditions; with some named as oxygen-regulated proteins (ORPs). These bind to unfolded / misfolded proteins to facilitate correct assembly. ORP 150 is the subject of this study. Hypoxia results in an enhancement of ORP 150 expression in several tumour cell lines cultured in vitro. HeLa cells grown in hypoxic conditions (despite an intensive expression of ORP 150) demonstrate higher rates of apoptosis in comparison to those cultured in normoxic conditions. Furthermore, the inhibition of ORP 150 synthesis by transfection of these cells with a specific siRNA resulted in an intensification of apoptosis, as indicated by specific markers of this process; the enhancement of poly ADP-ribose protein cleavage and the increase in Bim protein expression. We conclude from our study that the increase in ORP 150 synthesis protects the cells against the proapoptotic effect of hypoxia.

Cooperation of ER-60 and BiP in the Oxidative Refolding of Denatured Proteins In Vitro

ER-60 is a PDI family protein that has protein thiol-disulfide oxidoreductase activity. It has been shown to associate with BiP in the endoplasmic reticulum. Here, we analyzed the cooperation of ER-60 and BiP in the oxidative refolding of denatured proteins in vitro. ER-60 facilitated the refolding of 20 or 30% of denatured alpha-lactalbumin or ribonuclease B during incubation for 80 min, and these levels of nearly doubled on the addition of BiP to the reaction mixture. BiP alone could not refold denatured ribonuclease B, but could refold denatured alpha-lactalbumin a little. Enhancement of oxidative refolding of alpha-lactalbumin by ER-60 could be detected only when ER-60 was present from the start of refolding. On surface plasmon resonance analysis, ER-60 was shown to associate with BiP. The association was not influenced by ATP or ADP. Domains a and/or b' of ER-60 were implicated in the association with BiP.

Glucose effect on the expression of 150kDa oxygen-regulated protein in HeLa cells

Chaperones assist in the correct folding of newly synthesised proteins in the endoplasmic reticulum (ER) of cells, this being essential for the translocation of protein molecules to specific subcellular compartments, extracellular matrix or to biological fluids. The biosynthesis of some ER chaperones is regulated by glucose. They are named "glucose-regulated proteins" (GRPs). The function of some GRPs depends on oxygen, a subgroup named "oxygen-regulated proteins" (ORPs). The biosynthesis of ORPs is induced by deprivation of glucose or oxygen. Exposure of HeLa cells to glucose starvation induces the biosynthesis of various GRPs including ORP 150. The expression of ORP 150 is regulated by the concentration of glucose in the culture medium, being induced by a shortage and repressed by a presence of glucose. We have shown that both glucose starvation and transfection of cells with siRNA (specific to ORP 150 mRNA) evoke similar, but quantitatively different, effects. The cells grown for 72 h in a 4.5 mg/ml glucose-containing medium demonstrated low apoptosis (3.7%) whereas in a 0.5 mg/ml glucose-containing medium the apoptosis was increased to 10%. The effect of transfection on apoptosis was distinctly higher with almost 22% of apoptotic cells detected in 72 h cultures. One may conclude that ORP 150 reduces the pro-apoptotic effects of glucose starvation. Such a hypothesis is supported by the observation that the transfection procedure makes HeLa cells resistant to the regulatory effect of glucose on ORP 150 production. The transfected cells do not respond to glucose starvation with an overexpression of ORP 150. It is apparent from our experiments that ORP 150 plays an important role in adaptation of cells to the shortage of glucose and reduces the pro-apoptotic effect of glucose starvation.

Expression clustering reveals detailed co-expression patterns of functionally related proteins during B cell differentiation: a proteomic study using a combination of one-dimensional gel electrophoresis, LC-MS/MS, and stable isotope labeling by amino acids in cell culture (SILAC)

B cells play an essential role in the immune response. Upon activation they may differentiate into plasma cells that secrete specific antibodies against potentially pathogenic non-self antigens. To identify the cellular proteins that are important for efficient production of these antibodies we set out to study the B cell differentiation process at the proteome level. We performed an in-depth proteomic study to quantify dynamic relative protein expression patterns of several hundreds of proteins at five consecutive time points after lipopolysaccharide-induced activation of B lymphocytes. The proteome analysis was performed using a combination of stable isotope labeling using [13C6]leucine added to the murine B cell cultures, one-dimensional gel electrophoresis, and LC-MS/MS. In this study we identified 1,001 B cell proteins. We were able to quantify the expression levels of a quarter of all identified proteins (i.e. 234) at each of the five different time points. Nearly all proteins revealed changes in expression patterns. The quantitative dataset was further analyzed using an unbiased clustering method. Based on their expression profiles, we grouped the entire set of 234 quantified proteins into a limited number of 12 distinct clusters. Functionally related proteins showed a strong correlation in their temporal expression profiles. The quality of the quantitative data allowed us to even identify subclusters within functionally related classes of proteins such as in the endoplasmic reticulum proteins that are involved in antibody production.

Shiga toxin is transported from the endoplasmic reticulum following interaction with the luminal chaperone HEDJ/ERdj3

Shiga toxin (Stx) follows a complex intracellular pathway in order to kill susceptible cells. After binding to cell surface glycolipids, the toxin is internalized and trafficked in retrograde fashion to the endoplasmic reticulum (ER). From the ER lumen, the toxin must gain access to the cytoplasm, where it enzymatically inactivates the 28S rRNA, inhibiting protein synthesis. The host molecules involved in this pathway and the mechanisms utilized by the toxin to access the cytoplasm from the ER are largely unknown. We found that Stx is capable of energy-dependent transport across the ER lumen, as has recently been demonstrated for the cholera and ricin toxins. Genetic screening for molecules involved in Shiga toxin trafficking yielded a cDNA encoding a prematurely truncated protein. Characterization of this cDNA revealed that it encodes a novel Hsp40 chaperone, designated HEDJ or ERdj3, localized to the ER lumen, where it interacts with BiP, a molecule known to be involved in protein retrotranslocation out of the ER. We demonstrated that within the ER lumen Stx interacts with HEDJ and other chaperones known to be involved in retrotranslocation of proteins across the ER membrane. Moreover, sequential immunoprecipitation revealed that Shiga toxin was present in a complex that included HEDJ and Sec61, the translocon through which proteins are retrotranslocated to the cytoplasm. These findings suggest that HEDJ is a component of the ER quality control system and that Stx utilizes HEDJ and other ER-localized chaperones for transport from the ER lumen to the cytosol.

ER stress and the unfolded protein response

Conformational diseases are caused by mutations altering the folding pathway or final conformation of a protein. Many conformational diseases are caused by mutations in secretory proteins and reach from metabolic diseases, e.g. diabetes, to developmental and neurological diseases, e.g. Alzheimer's disease. Expression of mutant proteins disrupts protein folding in the endoplasmic reticulum (ER), causes ER stress, and activates a signaling network called the unfolded protein response (UPR). The UPR increases the biosynthetic capacity of the secretory pathway through upregulation of ER chaperone and foldase expression. In addition, the UPR decreases the biosynthetic burden of the secretory pathway by downregulating expression of genes encoding secreted proteins. Here we review our current understanding of how an unfolded protein signal is generated, sensed, transmitted across the ER membrane, and how downstream events in this stress response are regulated. We propose a model in which the activity of UPR signaling pathways reflects the biosynthetic activity of the ER. We summarize data that shows that this information is integrated into control of cellular events, which were previously not considered to be under control of ER signaling pathways, e.g. execution of differentiation and starvation programs.

Protein folding and quality control in the endoplasmic reticulum

The endoplasmic reticulum (ER) is a highly versatile protein factory that is equipped with chaperones and folding enzymes essential for protein folding. ER quality control guided by these chaperones is essential for life. Whereas correctly folded proteins are exported from the ER, misfolded proteins are retained and selectively degraded. At least two main chaperone classes, BiP and calnexin/calreticulin, are active in ER quality control. Folding factors usually are found in complexes. Recent work emphasises more than ever that chaperones act in concert with co-factors and with each other.

ER chaperone functions during normal and stress conditions

Nearly all resident proteins of the organelles along the secretory pathway, as well as proteins that are expressed at the cell surface or secreted from the cell, are first co-translationally translocated into the lumen of the endoplasmic reticulum (ER) as unfolded polypeptide chains. Immediately after entering the ER, they are often modified with N-linked glycans, are folded into the appropriate secondary and tertiary structures, which are stabilized by disulfide bonds, and finally in many cases are assembled into multimeric complexes. These processes are aided and monitored by ER chaperones and folding enzymes. When cells experience conditions that alter the ER environment, protein folding can be dramatically affected and can lead to the accumulation of unfolded proteins in this organelle. This in turn activates a signaling response, which is shared among all eukaryotic organisms, termed the unfolded protein response (UPR). The hallmark of this response is the coordinate transcriptional up-regulation of ER chaperones and folding enzymes. A major role for the increased levels of chaperones and folding enzymes during conditions of ER stress is to provide the same functions they carry out during normal physiological conditions. This includes preventing unfolded and incompletely folded proteins from aggregating and promoting the proper folding and assembly of proteins in the ER. During conditions of ER stress, many proteins are unable to fold properly and the requirements for chaperones are therefore increased. However, more recently it has become clear that some ER chaperones are also involved in signaling the ER stress response, targeting misfolded proteins for degradation and perhaps even shutting down the UPR when the stress subsides. In addition, during some normal physiological conditions, like plasma cell differentiation where there is an increased demand in the secretory capacity of B cells, the levels of various ER chaperones are also up-regulated via at least part of the UPR pathway. In order to discuss these various functions of ER chaperones, we will begin with the roles of ER chaperones and folding enzymes during normal physiological conditions and then discuss their roles during ER stress.